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The Impossibility of Approximate Agreement on a Larger Class of Graphs

Author Shihao Liu

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Subject Classification

ACM Subject Classification
  • Theory of computation → Distributed algorithms
  • Theory of computation → Computability
Keywords
  • Approximate agreement on graph
  • wait-free solvability
  • triangulated sphere

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Abstract

Approximate agreement is a variant of consensus in which processes receive input values from a domain and must output values in that domain that are sufficiently close to one another. We study the problem when the input domain is the vertex set of a connected graph. In asynchronous systems where processes communicate using shared registers, there are wait-free approximate agreement algorithms when the graph is a path or a tree, but not when the graph is a cycle of length at least 4. For many graphs, it is unknown whether a wait-free solution for approximate agreement exists.
We introduce a set of impossibility conditions and prove that approximate agreement on graphs satisfying these conditions cannot be solved in a wait-free manner. In particular, the graphs of all triangulated d-dimensional spheres that are not cliques, satisfy these conditions. The vertices and edges of an octahedron is an example of such a graph. We also present a family of reductions from approximate agreement on one graph to another graph. This allows us to extend known impossibility results to even more graphs.

Cite As Get BibTex

Shihao Liu. The Impossibility of Approximate Agreement on a Larger Class of Graphs. In 26th International Conference on Principles of Distributed Systems (OPODIS 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 253, pp. 22:1-22:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023) https://doi.org/10.4230/LIPIcs.OPODIS.2022.22

Author Details

Shihao Liu
  • Department of Computer Science, University of Toronto, Canada

Funding

This work is supported by the Natural Science and Engineering Research Council of Canada.

Acknowledgements

I want to thank my advisor Faith Ellen for her advice and many helpful discussions. I would also like to thank the anonymous reviewers for their helpful comments.

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